Phospholipids are a major component of biological membranes and the fluid mosaic model proposes that a phospholipid bilayer constitutes the basic unit of biological membrane structure. This phospholipid bilayer comprises a structural matrix of biological membranes with other membrane components either integral or peripheral to the bilayer. Phospholipids are an important component in such membrane functions as the constitution of a barrier to permeability and the activation of membrane-bound enzymes. The physical properties of phospholipids in the bilayer, their spatial arrangement, and their interactions with membrane proteins have been studied extensively. Their structural and functional importance in biological membranes has also generated considerable interest in the regulation and metabolism of various phospholipid species. Consequently many investigations of the control of membrane assembly have focused on the control of phospholipid synthesis and the relationship between phospholipid synthesis and membrane structure and function.
The biosynthetic pathway for phospholipids of Escherichia coli has been elucidated by developing in vitro enzyme assays for these activities. These investigations have established that phosphatidylethanolamine (PE) is synthesized from CDP-diglyceride through the sequential activities of phosphatidylserine synthetase and phosphatidylserine decarboxylase. Phosphatidylglycerol (PG) is the product of a separate pathway which diverged at the level of CDP-diglyceride. The study of phospholipid metabolism has been further facilitated by selection of temperature-sensitive mutant strains defective in phospholipid synthesis, with any alterations manifested in the accumulation of known pathway products and intermediates. For example, in a phosphatidylserine synthetase deficient mutant, cardiolipin increased and the amount of PE was reduced, while phosphatidylserine (PS) accumulated in a phosphatidylserine decarboxylase defective strain. However, it has been recently reported that a strain virtually devoid of PG has elevated levels of glycolipids, indicating an adjustment in cellular metabolism in response to the membrane phospholipid imbalance. The application of recombinant DNA techniques has allowed the construction of E. coli strains which overproduce phospholipid biosynthetic enzymes, but these strains do not possess altered cellular phospholipid compositions.
Physiological investigations of phospholipid enzymology have shown that it is restricted to the inner membrane of gram negative bacteria. Unfortunately, little has been reported on the localization of the phospholipid biosynthetic enzymes in the photosynthetic bacteria which possess an extensive intracytoplasmic membrane system (ICM).
The facultative photoheterotrophic bacterium Rhodopseudomonas sphaeroides has provided an attractive system in which to study membrane biogenesis and differentiation. When growing chemoheterotrophically, R. sphaeroides contains a typical gram-negative outer membrane and a cytoplasmic membrane. However, photoheterotrophic growth conditions induce the differentiation of the cytoplasmic membrane, resulting in the synthesis of an intracytoplasmic membrane system which houses the photosynthetic apparatus of the cell. Studies on the regulation of intracytoplasmic membrane assembly, employing synchronously dividing populations of R. sphaeroides, have shown that, while insertion of protein and photopigments into the intracytoplasmic membrane occurs continuously throughout the cell cycle, accumulation of phospholipids within the intracytoplasmic membrane occurs discontinuously with respect to the cell cycle. This discontinuity in phospholipid incorporation results from the bulk transfer of phospholipids from outside the intracytoplasmic membrane into the intracytoplasmic membrane concurrent with cell division.
Extensive studies on the growth of R. sphaeroides have led to the identification of a new phospholipid, normally present in minor proportions. The major phospholipid species generally found are PG, PE, and phosphatidylcholine (PC). It has now been found that varying the composition of the culture medium can surprisingly affect the production of particular phospholipids such as N-acylphosphatidylserine (NAPS).